Flood Damage in the United States, 1926-2003 A Reanalysis of National Weather Service Estimates

Executive Summary

Flood damage continues to increase in the United States, despite extensive flood management efforts. To address the problem of increasing damage, accurate data are needed on costs and vulnerability associated with flooding. Unfortunately, the available records of historical flood damage do not provide the detailed information needed for policy evaluation, scientific analysis, and disaster mitigation planning.

This study is a reanalysis of flood damage estimates collected by the National Weather Service (NWS) between 1925 and 2000. The NWS is the only organization that has maintained a long-term record of flood damage throughout the U.S. The NWS data are estimates of direct physical damage due to flooding that results from rainfall or snowmelt. They are obtained from diverse sources, compiled soon after each flood event, and not verified by comparison with actual expenditures. Therefore, a primary objective of the study was to examine the scope, accuracy, and consistency of the NWS damage estimates to improve the data sets and offer recommendations on how they can be appropriately used and interpreted.

This report presents the following three data sets:

Estimated flood damage in the U.S. (1926-1979 and 1983-2000, by fiscal year;

Estimated flood damage for each state in the U.S. (1955-1979, by calendar year, and 1983-2000, by fiscal year); and

Estimated flood damage, by river basin, for the U.S. (1933-1975, by calendar year).

We found that the NWS collection and processing of flood damage data were reasonably consistent from 1934 to the present, except during the period 1976-1982. Data from NWS files and other sources made it possible to reconstruct state and national flood damage estimates for 1976-1979. However, little data was collected during 1980-1982 and large errors were discovered in estimates developed later for that period. As a result, the years 1980-1982 are excluded from the reanalyzed data sets.

Evaluation of the accuracy of the estimates led to the following conclusions:

Individual damage estimates for small floods or for local jurisdictions within a larger flood area tend to be extremely inaccurate. When damage in a state is estimated to be less than $50 million (in 1995 dollars), estimates from NWS and other sources frequently disagree by more than a factor of two.

Damage estimates become more accurate at higher levels of aggregation. When damage in a state is estimated to be greater than $500 million, disagreement between estimates from NWS and other sources are relatively small (40% or less). The relatively close agreement between NWS and state estimates in years with major damage is reassuring, since the most costly floods are of greatest concern and make up a large proportion of total flood damage.

Floods causing moderate damage are occasionally omitted, or their damage greatly underestimated, in the NWS data sets. Missing NWS estimates were discovered for floods in which the state claimed as much as $50 million damage.

In summary, the NWS flood damage estimates do not represent an accurate accounting of actual costs, nor do they include all of the losses that might be attributable to flooding. Rather, they are rough estimates of direct physical damage to property, crops, and public infrastructure. Estimates for individual flood events are often quite inaccurate, but when estimates from many events are added together the errors become proportionately smaller.

At the national level, these findings suggest that annual damage totals are reasonably accurate because they are sums of damage estimates from many flood events. State annual damage estimates are more problematic. Both frequency and magnitude of damage must be considered, because damaging floods do not occur every year in most states. Flood frequency cannot be determined simply by the presence or absence of a damage estimate because reporting, particularly for small floods, is unreliable.

Aggregation is a key to reducing estimation errors. To compare flood damages between states, aggregate the damage estimates over many years and compare the sums. To compare damage between years, aggregate yearly state damage estimates over multi-state regions. Even when the estimates are highly aggregated, be aware that a substantial amount of variability is caused by estimation errors and interpret the results accordingly.

When properly used, the reanalyzed NWS damage estimates can be a valuable tool to aid researchers and decision makers in understanding the changing character of damaging floods in the United States. Users of the reanalyzed data are advised to take the following precautions:

To compare flood damage over time, adjust for changes in population, wealth, or development.

To compare damage in different geographical areas, control for differences in population and in the incidence of extreme weather events during the period of study.

Use damage estimates for individual floods with caution, recognizing that estimation errors are large. Comparison of individual floods might be better done using nominal or ordinal damage levels. Look for qualitative descriptions to compare the nature and impacts of the damage.

Different agencies define "flood" and "flood damage" somewhat differently. Check for incompatibilities between data from different sources before seeking to combine sources or aggregate data.

The NWS damage estimates are not reliable enough to be a basis for critical decisions, such as setting flood insurance premiums or evaluating the cost-effectiveness of specific hazard mitigation measures. Better damage data are needed to evaluate the effectiveness of specific mitigation measures designed to reduce flood losses.